Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element). An inorganic electroluminescent element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element. An organic electroluminescent element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence•phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
However, in an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption.
In Japanese Patent No. 3093796, a slight amount of a fluorescent substance has been doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element.
Further, there are known such as an element having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, JP-A 63-264692) and an element having an organic emitting layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, JP-A 3-255190).
In the case of utilizing emission from an excited singlet as described above, since a generation ratio of a singlet exciton to a triplet exciton is ⅓, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of an external quantum efficiency (ηext) of taking out light is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (M. A. Baldo et al., Nature vol. 395, pp. 151-154 (1998)), researches on materials exhibiting phosphorescence at room temperature have come to be active.
For example, it is also disclosed in A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), and U.S. Pat. No. 6,097,147.
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, in such as S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied.
Further, in the aforesaid, A. Baldo et al., Nature, vol. 403, No. 17, pp. 750-753 (2000), utilization of tris(2-phenylpyridine)iridium as a dopant has been studied.
In addition to these, M. E. Tompson et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied to utilize L2Ir(acac) such as (ppy)2Ir(acac) as a dopant, Moon-Jae Youn. Og., Tetsuo Tsutsui et al., also at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu), have studied utilization of such as tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3) and tris(benzo[h]quinoline)iridium (Ir(bzq)3) (herein, these metal complexes are generally referred to as orthometalated iridium complexes).
Further, in also the aforesaid, S. Lamansky et al., J. Am. Chem. Soc., vol. 123, p. 4304 (2001), studies have been carried out to prepare an element utilizing various types of iridium complexes.
Further, to obtain high emission efficiency, Ikai et al., at The 10th International Workshops on Inorganic and Organic Electroluminescence (EL'00, Hamamatsu) utilized a hole transporting compound as a host of a phosphorescent compound. Further, M. E. Tompson et al. utilized various types of electron transporting materials as a host of a phosphorescent compound doped with a new iridium complex. An orthometalated complex provided with platinum instead of iridium as a center metal is also attracting attention. With respect to these types of complexes, many examples having a characteristic ligand are known (for example, refer to Patent Documents 1-5 and Non-Patent Document 1).
In any case, emission luminance and emission efficiency are significantly improved compared to conventional elements because the emitting light arises from phosphorescence, however, there has been a problem of a poor emission lifetime of the element compared to conventional elements. It is hard to achieve an emission of a short wavelength and an improvement of an emission lifetime of the element for a phosphorescent emission material provided with a high efficiency. At present state, it cannot be achieved a level of a practical use.
In order to improve the above-described defects, there are known an Ir complex and a Pt complex having a phenylimidazole ligand (for example, refer to Patent Documents 6-7). However, the emission efficiency and the emission lifetime are still not fully satisfied, and further improvement in emission efficiency and emission lifetime are demanded.
[Patent Document 1] JP-A 2002-332291
[Patent Document 2] JP-A 2002-332292
[Patent Document 3] JP-A 2002-338588
[Patent Document 4] JP-A 2002-226495
[Patent Document 5] JP-A 2002-234894
[Patent Document 6] WO 02/15645
[Patent Document 7] WO 05/7767